H.E.S.S.

High Energy Stereoscopic System

HESS J1912+101 - Yet Another Pulsar Wind Nebula?

May 2007

Illustration of the Milky Way with its spiral
arms, showing the location of the sun. The initial H.E.S.S. survey of
the Galactic plane covered the range between +30 and -30 degr.
longitude. More recently the survey was extended beyond these limits,
leading to the discovery - among other objects - of HESS J1912+101 (in
the direction indicated by the dashed line). Credit:
R. Hurt (SSC),
JPL-Caltech, NASA.

The H.E.S.S. survey of the Galactic Plane (Aharonian et
al.
2005,
2006) initially covered the range from -30 degr. to +30 degr.
Galactic longitude. More recently, the survey region was extended to cover more than
twice the longitude range, out +60 degr. In this newly covered region, the
number of new gamma-ray sources is somewhat smaller than in the original survey region,
which can be attributed both to the reduced amount of mass along the line of
sight (top figure) and to the fact that larger positive longitudes are observed
at increasingly unfavorable zenith angles, reducing the sensitivity of the
instrument. One of the new sources is HESS J1912+101, located at l=44.4 degr.,
b=-0.1 degr.

Fig. 1 shows a smoothed gamma-ray
map of the region around longitude l=44.5 degr. HESS J1912+101 is visible as an extended
excess of gamma rays. The source was initially detected in a search for slightly
extended sources, summing up gamma-rays within a radius of 0.22 degr. from a
given sky position - the same cut as used in the published survey. A
gamma-ray excess of 7.7 sigma was visible, or about 5.7 sigma
taking into account the trails factor accounting for the large number of sky
positions probed in the survey. The source is clearly extended with an intrinsic rms source size of about 0.25 degr. With a larger integration radius of 0.46
degr. - the dashed circle in Fig. 1 - the significance
increases to 8.4 sigma (pre-trials), with a signal of 337 gamma rays. The total gamma ray flux is
about 10% of the flux of the Crab Nebula.

Close to the center of HESS J1912+101 is the pulsar PSR
J1913+1011 (Manchester
et al., 2005), which suggests that HESS J1912+101 might be powered by an
outflow of highly relativistic electrons and positrons from the pulsar, generating an extended pulsar wind nebula (PWN).
With a spin-down luminosity of 2.9 x 1036 ergs/s and a distance of
about 4-5 kpc, less than 1% of the pulsar's spin-down power would be needed to
account for the gamma-ray flux, a value well within the range observed for
other PWN. The relatively large size of the source could be attributed to the
age of 1.7 x 105 years of the pulsar. The lack of spherical symmetry
of the nebula
is a characteristic of many gamma-ray PWN and X-ray PWN (e.g.
SOM 9/2005). A search for other possible
counterparts reveals a faint ROSAT X-ray source (Fig. 1) (Voges
et al. 2000)
co-located with an infrared source (Fig. 2) as well as
indications of molecular clouds, which are seen as an
enhancement in 13CO emission in the velocity range corresponding to
the pulsar distance, with the H II region G044.3+00.1 (Paladini
et al. 2003) (Fig. 3). The presence of this
target gas and dust allows an alternative interpretation of gamma-ray production in terms of proton
acceleration and interactions.

References:

H.E.S.S.
collaboration, F. Aharonian et al., in
preparation

Fig. 1:
The VHE gamma-ray source HESS J1912+101; shown is a smoothed gamma-ray sky
map, with the locations of the pulsar PSR J1913+1011 (open square), of the
faint ROSAT X-ray source (circle) and of the nearby Integral gamma-ray
source IGR J19140+0951(triangle,
Hannikainen et al. 2004). The dashed line is the Galactic equator.
Preliminary.

Fig. 2:
Infrared image at 8.3 microns of the same region as in Fig. 1, from the
MSX
instrument. White lines give gamma-ray significance contours, corresponding
to the black lines in Fig. 1. Preliminary.